GPU function in Blender
GPU can be used for faster viewport display, more vertices count. A PC with a powerful GPU can handle Millions of vertices and faces, but Consumer Notebooks cannot be used to get that performance. GPU can also take over some functions of CPU in rendering, it can handle Volumes , Procedural Textures better than CPU, specifically GTX is mostly used for Realtime, and is famous among gamers, and RTX is for  has specific algorithms for raytracing, (useful for Cycles in blender). When you set Your GPU in the CUDA section in Preferences , you let the GPU do most of the calculations in blender(after setting to GPU Compute in Render settings) and it can do somethings faster than your CPU.